CO₂ Capture by Na₂TeO₄: Structure of Na_2–xH_xTeO₄ and Kinetic Aspects

β-Na₂TeO₄ is able to trap CO₂ in a humid atmosphere due to a partial Na⁺/H⁺ exchange and the formation of NaHCO₃. The RT powder X-ray diffraction pattern of the resulting Na_2–xH_xTeO₄ shows broad and narrow hkl lines preventing the structural study. We show by the DIFFaX program that Na⁺/H⁺ exchange is topotactic since the structure, as in the mother form, consists of [TeO₄]_n^2n– chains of TeO₆ octahedra. We also show that the broadening of some hkl lines is due to stacking faults which result from the weakness of H–O···H bonds connecting the [TeO₄]_n^2n– chains. Upon heating, a progressive structural organization takes place which has been followed by powder X-ray diffraction, Raman, and NMR spectroscopies. Around 300 °C, a well organized structure can be described from powder X-ray diffraction refinements in the monoclinic P2₁/n space group while ab initio computations allowed location of the hydrogen atoms with satisfactory H–O bonds. In addition, we present the CO₂ sorption/desorption by Na₂TeO₄ and compare its performance to that of Na₂SiO₃. Finally, the existence of a Na_2–xLi_xTeO₄ solid solution (0 ≤ x ≤ 0.9) is evidenced, and we show that the presence of lithium in the structure leads to the disappearance of the structural transition observed for β-Na₂TeO₄ and to a progressive decrease of the CO₂ capture ability.